This is my first time writting, I'm going through alot and I feel like I am about to lose it.

I started dating my now finance 2 years ago, he has 3 kids, twins - boy(5) girl(5) and another boy(7). When I first dating my fiance, I didn't spend much time with the kids as I wanted to keep my distance incase things didn't work out. Fast forward 2 years we are now living together. When I came into the picture the kids had no manners or rules, they didn't do any chorus(appropriate for their age). It drove me crazy so I quickly changed that, they now use their manners and do chorus at my house but as soon  as they come back from their BM, it's like nothing ever happened and I have to start over. 

THe youngest boy had to go to the hospital for stitches one day(this was on her week), of course I went but I stayed in the waiting room. While we were all the there (the kids, my fiance, and the BM) the kids we were running around the waiting room, screaming, climbing the chairs, rumiging through her purse and taking money out of it and running around with it. MY SD was screaming so I went up to her and I asked her if she could please sit down and be quiter as we are in a waiting room and this isn't the place to be running around and screaming. The BM looked at me and said "you can't ask a 5 year old to sit down and be quiet". Obviously I wasn't expecting her to do it for hours on end but thats the time to show them proper public ediquet. I was fuming that she said that to me. She made me feel like I was in the wrong for saying that to my SD

The oldest son is not my fiance biological son When he started dating BM she had a 1 year old son. Then BM got pregnant with my fiances twins. Long story short he took the oldest son as his own because his real dad didn't want anything to do with him. 

Fast forward to now we have all the kids week on week off - BM gets child support for the oldest son from his BD, but my fiance doesn't get any of that money. is this fair? Last year the oldest started grade 1. We got multiple notes from the teacher stating that he is having alot of trouble focusing in school and is disruptive to the other students. I noticed behaviors at home to, he gets frustrated over little things and will freak out and hit himself. I believe he has ADHD but his BM thinks all of that is normal behavior. I'm unsure? 

My Fiance does not pay child support however BM has make threats about it so they came to the agreement that he will pay for all 3 of the kids day care, even though she gets money from the oldest BD.

Here's my real problem. BM has been in 4 different relationships(I use that term loosely in her case) 1 of the men shes been with had 3 other kids. 1 of the guys she beens with was an alcoholic, and this last guy is the real kicker - she told my fiance that he was mentally abusive to her, is biopolar and has fetal alcohol symdrom. They dated for 2 weeks then got engaged and moved in together, she is now 4 months pregnant with his child. They are no longer together. The children has told me multiple times that they fight alot. BM has told us that they night she broke up with him he really scared her and the kids because he broke her door in half and was breaking dishes and throwing stuff at her. After that night it was our day to get the kids and they told us that their BM and that guy got in a fight in the car while they were in it and she dumped coffee on her and then she throw coffee in his face. The next morning my SD woke up screaming - I went to her to see what was going on, she expressed that she had a bad dream, I asked her what about and she said "someone stabbed mom and killed the baby" since then every night she mentions that dream and she says she hopes it doesn't happen again. ALL the kids also mentioned multiple times about the fight in the car.

She crosses boundaires in my opinion with my fiance - she texted him the other day my fiance to see if she gave him money if he could take the kids to get her a baby shower present. She has also asked us to watch the kids while she is in labor. He never tells her no - I think hes scared of her. My fiance and I never ever fight, we have very good communication but this is the one thing that has drove a wedge between us. 

I feel like that is crossing the line - Her decision to have another child is not our responsibility. 

I'm at my wits end with all of this, I don't know how much longer I can deal and I feel really defeated. I may be to invested and I think I need to learn where to step back but I'm having a really hard time. 

I have talked to friends and family about all of this but I feel like I need to hear from another step parent.